1. Field of the Invention
The present invention relates to data reading methods and semiconductor memory devices, particularly to ferroelectric memories using oxide ferroelectric thin films for capacitors.
2. Description of the Related Art
In recent semiconductor memory devices used is a ferroelectric memory called an FRAM in place of a volatile memory such as a DRAM. The ferroelectric memory is a memory using a ferroelectric thin film (PZT film, PLZT film, BTO film, or the like) for a storage element and has such excellent characteristic features that the times of rewrite can be greatly increased, the energy necessary for a rewrite can be reduced, and the rewrite speed can be considerably increased.
There is a ferroelectric memory having such a ferroelectric capacitor integrated on a semiconductor device.
The ferroelectric memory can be formed with almost the same structure as that of a DRAM except that a ferroelectric film is used as the capacitor film (dielectric film) of a memory cell and allows high degree of integration like a DRAM. In addition, since polarization inversion of a ferroelectric material is used, written data is nonvolatile, and can be rewritten at high speed and low power consumption.
The structure and operation of the ferroelectric memory will be described below with reference to drawings.
FIG. 1 shows the hysteresis characteristics of the ferroelectric capacitors C0 and C1. Referring to FIG. 1, the abscissa represents the potential of the electrode on the plate line PL side with respect to that of the electrode on the bit line BL0 or BL1 side, and the ordinate represents polarization. For the illustrative convenience, points +Pr and xe2x88x92Pr represent the states xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d, respectively.
As a 2-transistor/2-capacitor memory cell, data xe2x80x9c1xe2x80x9d corresponds to a state wherein xe2x80x9c0xe2x80x9d is written in the ferroelectric capacitor C0, and xe2x80x9c1xe2x80x9d is written in the ferroelectric capacitor C1, and data xe2x80x9c0xe2x80x9d corresponds to a state wherein xe2x80x9c0xe2x80x9d is written in the ferroelectric capacitor C1, and xe2x80x9c1xe2x80x9d is written in the ferroelectric capacitor C0.
FIG. 2 is a timing chart showing circuit operation in reading data xe2x80x9c1xe2x80x9d. This circuit operation in reading data xe2x80x9c1xe2x80x9d will be described below with reference to FIG. 2.
First, the potential of the word line connected to the selected cell is raised to turn on the switching transistors. Next, a pulse is input to the plate line PL. Charges move onto the bit line BL1 due to polarization inversion of the ferroelectric capacitor C1, so the potential of the bit line BL1 rises.
On the other hand, the potential of the bit line BLO does not change because no polarization inversion occurs in the ferroelectric capacitor C0. When the sense amplifier 1 is activated (high state) time t0xe2x80x2 after from the pulse input, the potential difference between the bit lines BLO and BL1 is amplified, and data is externally read out. At this time, the data is written in the ferroelectric capacitor C1 again due to the potential difference between the plate line PL and the bit line BL1. Next, a pulse is input to the plate line PL again, thereby writing opposite data in the ferroelectric capacitor C0.
However, when data are written in the ferroelectric capacitors C0 and C1 and held for a long time, a voltage shift occurs in hysteresis characteristics. This phenomenon is called an imprint effect. When imprinting occurs, polarization in one direction becomes stable. However, when opposite data is written, depolarization occurs to make the data write/read difficult.
FIGS. 3A and 3B, and 4A and 4B show changes in polarization in reading data from an imprinted ferroelectric capacitor. Symbol xcex94P01 and the like in FIGS. 3A to 4B represents a polarization change amount, in which the first numeral indicates the direction of imprinting, and the second numeral indicates data to be read. For example, xcex94P01 means that xe2x80x9c1xe2x80x9d is written/read in/from a capacitor imprinted in the xe2x80x9c0xe2x80x9d direction. A write/read of data in the same direction as the imprinting direction is represented by SS (Same State), and a write/read of data in the direction opposite to the imprinting direction is represented by OS (Opposite State). Hence, xcex94P01 and xcex94P10 correspond to OS (Opposite State), and xcex94P00 and xcex94P11 correspond to SS (Same State).
Referring to FIGS. 3A and 3B, in the OS (Opposite State), when imprinting progresses, the value xcex94P01 decreases. On the other hand, as shown in FIGS. 4A and 4B, in the SS (Same State), the value of polarization change rarely changes. For this reason, when the potential difference between the bit lines BL0 and BL1 is to be detected in the OS (Opposite State), the difference between the value xcex94P01 and the value xcex94P10 is detected as a smaller value than in an un-imprinted state because the value xcex94P01 decreases and the value xcex94P10 increases. As imprinting progresses, the difference between the value xcex94P01 and the value xcex94P10 decreases.
FIGS. 5A and 5B show a change over time in each polarization. FIG. 5A shows a signal margin in the SS (Same State). FIG. 5B shows a change in signal margin in the OS (Opposite State). As shown in FIG. 5A, in the SS (Same State), since the polarization change amount does not vary due to imprinting, the signal margin does not change. However, in the OS (Opposite State), the signal margin changes over time as imprinting progresses. When this margin becomes lower than the read capability of the circuit, the data read is disabled. This determines the service life of a device.
Such an imprint effect is an inherent phenomenon caused by the hysteresis characteristics of a ferroelectric film and therefore cannot be completely suppressed. This imprinting makes it impossible to read desired data and also limits the service life of a device.
As described above, an FRAM has excellent characteristics that nonvolatile data can be written, and can be rewritten at high speed and low power consumption. However, when a shift due to the imprint effect occurs in the hysteresis characteristics of the capacitor, a data read error occurs, and the service life is shortened.
It is an object of the present invention to provide a data reading method and a semiconductor memory device, which allow a data read with a minimum read error by ensuring an output margin necessary for the read and improve the service life of a device even when imprinting occurs in a ferroelectric memory using a ferroelectric film and the polarization change amount varies.
According to the present invention, there is provided a data reading method of reading data stored in a ferroelectric capacitor having one electrode connected to a plate line and the other electrode connected to a bit line through a selecting transistor, by inputting a pulse to the plate line and then performing sense operation to amplify the data. The sense operation is performed after a signal is output from the bit line on the basis of the pulse, and the signal output is decreased from a peak value.
According to another aspect of the data reading method of the present invention, the signal output is decreased by ensuring a predetermined time from input of the pulse to the sense operation.
According to the present invention, there is also provided a semiconductor memory device for reading stored data by the above-described data reading method, wherein a data read or write is performed while an imprint effect of the ferroelectric capacitor is reduced in a refresh or every predetermined time or every predetermined number of times of read.
The present invention comprises the above technical means. Even when imprinting occurs in the hysteresis of a ferroelectric film formed in the ferroelectric memory, and the signal output from the non-switching capacitor (data xe2x80x9c0xe2x80x9d) becomes larger than a normal value, unfavorable increase of bit line level can be canceled because the sense operation is performed after the signal output is decreased from the peak value. Hence, even when imprinting occurs in a data write or read, the signal output margin can be ensured.
According to the present invention, even when the characteristics of the ferroelectric capacitor change due to imprinting, data can be accurately read. Hence, high reliability and long service life of a device can be realized.